1. Field of the Invention
This invention relates to a programmable ROM (Read Only Memory).
2. Description of the Related Art
Conventionally, as a programmable ROM, a poly-fuse (a fuse which is comprised of a poly-silicon) and PCOP (Pure CMOS one-time programmable) memory are known.
For example, the poly-fuse stores data by causing a large current to pass through the poly-fuse, converting the same into an amorphous form and changing the resistance thereof. Further, for example, the PCOP memory stores data by applying high voltage to the gate insulating film of a P-channel MOS transistor and destroying the same. In either situation, since data can be programmed only once, the above memories are called an OTP (One time programmable ROM).
However, in the above programmable ROM, since a large current or high voltage is required for data programming, the configuration of a control circuit that controls the data programming becomes complicated.
Generally, the technique for using an SRAM (Static RAM) as a ROM is known (for example, refer to Jpn. Pat. Appln. KOKAI Publication No. H6-112437 and Jpn. Pat. Appln. KOKAI Publication No. H7-130180).
The principle is that the resistances of the load resistors of the SRAM, that is, the load resistors of back-to-back inverters are made different to set the initial state at the power-ON time to a fixed value.
However, the resistances of the load resistors are determined by a dose amount in ion implantation in a wafer process. In this situation, the number of mask processes is increased and this enhances the manufacturing cost.